1. Field of the Invention
The present invention relates to an RF transceiver, and more particularly, to a multi-mode & multi-band RF transceiver, like a GSM/EDGE & WCDMA dual-mode & multi-band cellular phone, and related communications method.
2. Description of the Prior Art
The past decade has shown an explosive growth in wireless communications systems. A variety of communications systems, such as GSM and CDMA, have been introduced to the market of cell phones to realize wireless communications functions. A cell phone comprises a wireless RF transceiver to transmit/receive wireless signals. An RF transceiver usually comprises a phase-locked loop (PLL) as a frequency synthesizer to generate a carrier signal for a local oscillator (LO).
In general, a typical multi-mode or multi-band RF transceiver has to comprise more than one LO, and more than one PLL accordingly, to generate more than one carrier signal for a variety of bands or system modes, therefore increasing the system complexity and the product cost due to an enlarged chip size.
An RF transceiver comprises a transmitter to emit wireless communications signals. Please refer to FIG. 1, which is a function block diagram of a wireless transmitter 10 according to the prior art. The basic function of the transmitter 10 is to modulate, or to decode baseband information, such as voice, video, data or other information, onto a high frequency sine wave carrier that can be radiated by a transmit antenna. The reason for this is that signals at higher frequencies can be radiated more effectively, and use the RF spectrum more efficiently, than the direct radiation of the baseband signals. The transmitter 10 comprises a first local oscillator (LO) 12 for generating a first LO signal, an I/Q modulator 14 for modulating an I/Q baseband signal with the first LO signal into an intermediate frequency (IF) signal of a frequency usually ranging from 10 to 100 MHz, a first bandpass filter 16 for passing frequency components within a narrow passband while rejecting frequency components like noises outside the passband, a second LO 18 for generating a second LO signal, a mixer 20 to up-convert the IF signal output from the first bandpass filter 16 into a sum and a difference of the IF signal and the second LO signal by mixing the IF signal with the second LO signal, a second bandpass filter 22 connected to the mixer 20 for passing the sum of the IF signal and the second LO signal only, a power amplifier 24 for increasing the power of signals output from the second bandpass filter 22, and a transmit antenna 26 for converting signals with amplified power from the power amplifier 24 to propagating electromagnetic place waves.
The first LO 12 and the second LO 18 both are made up of a PLL having a voltage-controlled oscillator installed. A feedback control circuit of the PLL enables the voltage-controlled oscillator to precisely track the phase of a stable reference oscillator. The two-staged transmitter 10, which is applied to WCDMA communications system, has advantages of reduced LO pulling, lower LO feedthrough, and milder cross-talk between I/Q channels.
An RF transceiver comprises not only a transmitter, but also a receiver. Please refer to FIG. 2, which is a function block diagram of a superheterodyne receiver 30 according to the prior art. The receiver 30 comprises an antenna 32, a third bandpass filter 34, a low noise amplifier 36 for amplifying possibly very weak received signals received by the antenna 32 while minimizing noise power that is added to the received signals, a third LO 38 for generating a third LO, a second mixer 40 to down-convert signals transmitted from the low noise amplifier 36 into an IF signal, a fourth bandpass filter 42 connected to the second mixer 40, a fourth LO 44 for generating a fourth LO, and a demodulator 46 connected to the fourth bandpass filter 42 and to the fourth LO 44 for recovering an I/Q baseband signal from signals filtered by the fourth bandpass filter 42.
According to FIG. 1 and FIG. 2, a transceiver for a cellular phone comprises four elaborate LOs.
Because of the very competitive nature of the cellular phone market, there is a strong demand to reduce the parts count, size, weight, and cost of the transmitter 10, and of the receiver 30 as well. Direct conversion transmitters, which are applied to GSM communications system, are therefore of significant interest, because the first LO 12, the first bandpass filter 16 and the second bandpass filter 22 of the transmitter 10 are eliminated here with this type of transmitter topology.
The above drawback can be overcome if a multi-mode or multi-band RF transceiver can share a single frequency synthesizer capable of generating a variety of carrier signals without impacting the quality of transmitting and receiving signals dramatically. Such an RF transceiver has a simple structure and a low cost. Please refer to FIG. 3, which is a function block diagram of a direct conversion transmitter 50 according to the prior art. A pair of I/Q signals enter a fifth bandpass filter 52 and a sixth bandpass filter 54 respectively and mix with an LO signal generated by a fifth LO (PLL) 56 for orthogonalization. The orthogonalized I/Q signals enter a power amplifier 58 for power amplification and are transmitted by a transmit antenna 60.
Despite having an advantage of comprising a minimum of components, the transmitter 50 may still suffer from a problem of LO injection pulling and must requires an additional topology for isolation. Additionally, the PLL, the mixer and the adder of the transmitter 50 have to be designed elaborately to have very low phase noise, therefore increasing the complexity to designing IC circuit and semiconductor manufacturing process.